Add volatile to a suppress warning.

Without the fix, the following warning is shown:

    compiling cont.c
    cont.c: In function ‘rb_callcc’:
    cont.c:949:39: warning: passing argument 1 of ‘cont_capture’ discards ‘volatile’ qualifier from pointer target type [-Wdiscarded-qualifiers]
	 volatile VALUE val = cont_capture(&called);
					   ^
    cont.c:473:1: note: expected ‘int * volatile’ but argument is of type ‘volatile int *’
     cont_capture(int *volatile stat)
     ^

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@57021 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

cont.c

@@ -161,7 +161,7 @@ static VALUE rb_eFiberError;
     if (!(ptr)) rb_raise(rb_eFiberError, "uninitialized fiber"); \
 } while (0)
 
-NOINLINE(static VALUE cont_capture(int *volatile stat));
+NOINLINE(static VALUE cont_capture(volatile int *volatile stat));
 
 #define THREAD_MUST_BE_RUNNING(th) do { \
 	if (!(th)->tag) rb_raise(rb_eThreadError, "not running thread");	\
@@ -470,7 +470,7 @@ cont_new(VALUE klass)
 }
 
 static VALUE
-cont_capture(int *volatile stat)
+cont_capture(volatile int *volatile stat)
 {
     rb_context_t *volatile cont;
     rb_thread_t *th = GET_THREAD();